The concept of applying a lubricating micropowder to printed images to enhance image permanence has been tried, with some measure of success. One such apparatus is described in U.S. Pat. No. 4,779,558. This patent shows a device for coating sheets of paper on which printed images have been applied, with one important aspect being to not apply so much powder as to prevent writing with a ball point pen while at the same time applying sufficient powder to reduce smearing of the printed images.
Some of the disadvantages of prior art micropowder application devices are that the micropowder is applied to the entire surface of the printed substrate, not just to the image areas. This results in a waste of the micropowder and thereby more rapid use of the supply and could interfere with writing on the non-printed areas of the substrate. Another disadvantage of the prior art is that the overcoating powder applied to the substrate and the images merely rest thereon as opposed to being adhered to the images and can thereby be dislodged during handling of the printed document, thereby reducing the effectiveness of the powder coating. On many documents only a certain portion of the printed areas are needed to have the image permanence enhanced but the prior art devices applies the same amount of powder to all of the printed areas, thereby needlessly consuming additional micropowder material and at the same time reducing the coefficient of friction on areas of documents which do not need image permanence protection, possibly interfering with their efficient handling, which normally requires that some element of friction be involved.
There are a number of application areas where enhanced image permanence can be beneficial. These include documents prepared for Magnetic Ink Character Recognition (MICR), machine readable bar codes and optical character recognition (OCR), photocopied and printed documents where it is desired to prevent chemical attack of toner images by, for example, the plasticizers contained in vinyl book covers, and other applications where it is useful to provide a protective barrier between printed indicia and sources of abrasion or chemical attack or both. A particular problem of non-impact printer produced documents are that they tend to transfer their image to vinyl material, such as notebook covers, especially when left exposed to a heat source, such as a car in the sunshine.
One of the more important areas where improvements are needed is where machine readable documents are automatically sorted. In a particular application where MICR readable toner is used to print financial documents such as checks, the smear problem can be significant. A serious limitation in utilizing non-impact printers to print MICR images has been the subsequent image smear as printed documents are processed through high-speed reader/sorters such as the IBM 3890. Documents having MICR images such as checks go through rapid automated clearing processes which often cause the image thereon to smear. The frictional forces involved in the handling of documents in reader/sorters, when opposed by the shear strength of the toner, results in some of the top portions of the toner actually tearing off. Some attempts in this area have been made to resolve the problem by developing a specialized MICR toner, more resistant to smearing. The problem with this solution is the tremendous development expense, coupled with the additional care that must be taken not to compromise base image quality, and the fact that the solution is machine specific. The costs for automatic sorting increases dramatically when character smear requires hand sorting. Just one or two percent unreadable documents can mean hundreds of millions of dollars in additional check sorting costs annually on a national basis.